Body Mass Index and Mortality

Recent research has shown that a BMI of 22.5 to 25.0 is associated with the lowest mortality rate. It has been known for some time that a lower BMI has been associated with an increased risk of death, but that was thought to be due mostly to smoking-related diseases. A 2009 meta-analysis of 900,000 people found that even in those who never smoked, there is a slight increase in mortality below a BMI of 22.5. (1)

The excess mortality below 22.5 has not been explained. One theory is that the excess mortality might be due to lower fat-free mass, which would most likely be lower muscle mass (though could also technically be bones, or even some organs). (1, 2) Studies on BMI and mortality to date have not differentiated between fat and fat-free body mass.

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Yes, that is how I read it. Remember that this study was only one finding, while the other study is a meta-analysis, so I would give the Prospective Studies Collaboration more weight. But that is very interesting.

> Isn’t there research out there on mortality and reduction of calories?

I’m not aware of any studies specifically looking at lower calorie intake and mortality in humans. I did a quick search at PubMed and didn’t find anything. I’m guessing that the researchers are using BMI as a proxy for caloric intake, although I can see how that might not be a good assumption. Studies of vegetarian mortality have included measurements of caloric intake, but I don’t recall one of them ever using the caloric intake of the individuals as the variable for measuring mortality rates.

http://www.nature.com/oby/journal/vaop/ncurrent/abs/oby2009191a.html
“A significant increased risk of mortality over the 12 years of follow-up was observed for underweight (BMI <18.5; relative risk (RR) = 1.73, P 35; RR = 1.36, P <0.05). Overweight (BMI 25 to <30) was associated with a significantly decreased risk of death (RR = 0.83, P 0.05). Our results are similar to those from other recent studies, confirming that underweight and obesity class II+ are clear risk factors for mortality, and showing that when compared to the acceptable BMI category, overweight appears to be protective against mortality. Obesity class I was not associated with an increased risk of mortality.”